1. Field of the Invention
This invention relates to a liquid crystal composition suitable as a liquid crystal material used for TN mode liquid crystal elements, particularly for multiplex drive mode display elements.
2. Description of the Related Art
In recent years, applications of liquid crystal display elements have been broadened from those in the fields of watches, electronic calculators, etc. toward those of various information processing terminals; thus the information display capacity has been extensively enlarged. One example is a multiplex drive employing voltage-leveling method. However, the number of liquid crystal materials usable for such a mode is small. Specific features generally required for liquid crystal compositions for display elements are as follows:
(A) the compositions are chemically stable and superior in weather resistance and have nematic liquid crystal properties within an operating temperature range in the vicinity of room temperature; PA0 (B) the temperature dependency of the voltage-transmittance characteristic is small and also the viewing angle is broad; PA0 (C) the voltage-transmittance characteristic is steep; etc. However, when liquid crystal display elements are subjected to a high multiplex drive, the specific feature (C) is particularly important among the abovementioned specific features. The specific feature (C) is an important factor directly related to the display contrast and to the level of multiplexing i.e. the visualizability and the information quantity of the cliquid crystal element. PA0 (1) 10 to 50% by weight of a first component comprising at least one member of compounds expressed by the formula ##STR4## wherein R.sub.1 represents an alkyl group of 1 to 8 carbon atoms, and (2) 20 to 90% by weight of a second component comprising
As well known, in the case of a multiplex drive mode utilizing a general voltage-leveling method wherein there is provided a cell provided with scanning electrodes and signal electrodes and having a liquid crystal material placed between these electrodes, the root-mean-square voltage of a selected element (Von) and that of a non-selected element (Voff) are expressed by the following equations: ##EQU1## wherein a represents the bias ratio, Vo represents the peak value of impressed voltage and N represents the number of scanning lines. As calculated from the conditions for maximizing the ratio of Von/Voff, the optimum bias ratio corresponds to .sqroot.N+1 and depends only on the number of scanning lines N. At that time, the following relationship exists between the operation margin .alpha. and the number of scanning lines: ##EQU2## and as the number of scanning lines is increased, the value of .alpha. rapidly approaches to 1. For example, if N=100, then .alpha.=1.1; thus this indicates that if the impressed voltage varies by about 10%, there occurs the so-called cross-talk where a non-selected element turns to the "on" state.
In order to prevent cross-talk from occurring, the following relationship may be sufficient to come into existence: EQU Voff&lt;Vth and Vsat&lt;Von .circle.4
wherein Vth and Vsat represent the threshold root-mean-square voltage and the saturation root-mean-square voltage in the voltage-transmittance characteristic of a liquid crystal display element, respectively.
In general, in the case of a multiplex drive, a root-mean-square voltage affording 10% of transmittance in the voltage-transmittance characteristic is often referred to as a threshold voltage and a root-mean-square voltage affording 50% of transmittance therein is often referred to as a saturation voltage. Thus, if the steepness of liquid crystals, .gamma., is equal to Vsat/Vth, a condition of .gamma..ltoreq..alpha. should be satisfied in order to obtain a display of good contrast. In other words, as the number of scanning lines increases, it is necessary that the .gamma. value of liquid crystals approach 1.